This invention generally relates to a photovoltaic roofing assembly, and in particular to a lightweight photovoltaic roofing assembly requiring no roofing penetrations and which resists wind up-lift due to specialized component geometry and by acting as an integral assembly.
As the cost of solar cells declines, the non-solar cell components necessary for a functioning photovoltaic system begin to dominate the overall system costs. For this reason, there is a growing trend to develop photovoltaic assemblies which eliminate or reduce non-solar cell components, and where the photovoltaic cell displaces conventional building components. Special care must be taken to ensure that new products based on photovoltaic materials remain safe with respect to environmental factors such as wind-loading and environmental stresses.
A prior art photovoltaic roofing assembly is shown in U.S. Pat. No. 4,886,554 issued Dec. 12, 1989 to Woodring et al. Woodring's assembly includes a plurality of insulation blocks disposed as a layer on top of a roofing membrane, a plurality of concrete pavers disposed as a layer on top of the plurality of insulation blocks, and a plurality of photovoltaic cells, each supported on a respective paver. A key feature of Woodring's assembly is the attachment of the solar cell to the supporting paver. But such attachment suffers from several disadvantages:
a) by including a roofing paver, the assembly is more complicated than necessary and more costly to manufacture.
b) the assembly does not employ a method by which to limit the temperatures experienced by the solar cells and other components. Solar cells are known to decline in efficiency with increasing temperatures. Hence, by offering no mechanism for temperature abatement, the assembly will operate less efficiently, with unknown long-term effects due to high temperature exposure.
c) by placing both a concrete paver and photovoltaic module onto the insulation block, the insulation block is inhibited from ventilating and expiring moisture. As a result, upon exposure to moisture, the insulation block takes longer to dry out, thus reducing its insulating value and degrading the integrity of the insulation block over time.
d) the assembly has multiple modes of potential failure, which include the paver component and its means of bonding. These components will be subjected to 20-30 years of an exposed and harsh weather environment at elevated temperatures. Any form of delamination is unacceptable. Delamination would cause dislocation of solar cells due to wind loading, and potential exposure of the insulation and membrane layers below.
Another prior art solar roofing assembly is shown in U.S. Pat. No. 4,674,244 issued Jun. 23, 1987 to Francovitch. Frankovitch's assembly includes a roof substrate which is substantially flat, an insulation structure thereon having an inclined surface, an elastomeric membrane over the substrate and the structure, the membrane being applied to and supported by the substrate and structure, and supporting an array of photocells. A key feature of this assembly is the attachment of the solar cell directly to the roofing membrane. By such attachment, this assembly suffers from several disadvantages:
a) the assembly does not employ a method by which to limit the temperatures that will be experienced by the solar cells and roofing membrane, thus reducing the efficiency of the solar cells and reducing the life of the roofing membrane.
b) the assembly has multiple modes of potential failure, which include failure due to thermal stresses on the roofing membrane and its means of bonding.
c) the assembly requires roof fasteners which penetrate the protective roofing membrane, which make the installation much more complicated and more costly than is necessary. In addition, such penetrations increase the risk of water leakage, with consequent damage to the building and its contents.
Other patents related to a photovoltaic roofing assembly include U.S. Pat. Nos. 4,835,918 issued Jun. 6, 1989 to Dippel; 4,189,881 issued Feb. 26, 1980 to Hawley; 3,769,091 issued Oct. 30, 1973 to Leinkram et al; 4,040,867 issued Aug. 9, 1977 to Forestieri et al; 4,321,416 issued Mar. 23, 1982 to Tennant; 4,860,509 issued Aug. 29, 1989 to Laaly et al; 5,092,393 issued March, 1992 to Nath et al; 5,112,408 issued May, 1992 to Melchior, 4,389,533 issued Jun. 21, 1983 to Ames; 4,677,248 issued Jun. 30, 1987 to Lacey; 5,338,369 issued Aug. 16, 1994 to Rawlings; German patent No. DE 3611542 A1 issued Apr. 5, 1986 to Cohausz et al.; and Japanese patent No. 3-200376 issued Sep. 2, 1991.